This invention relates to improving the retention of memory in a manner that may be power-efficient.
Many data processing applications make use of non-volatile or volatile memory storage, for example in the form of flash memory, DRAM (dynamic random access memory) or SRAM (static random access memory). In some types of memory energy must continually be supplied to the memory in order for it to maintain its state. In contrast, in non-volatile memory data can be stored when no power is applied. This makes non-volatile memory attractive for low-power applications. However, non-volatile memory can decay over time, resulting in the data it stores being unreliable. For example, in flash memory, data is stored through a remnant charge on a floating gate of a transistor. At room temperature that charge can be stable for many years. However, at elevated temperatures factors such as electron mobility are increased and so the rate of memory decay is greater than at room temperature. Hence, at higher temperatures the stored charge in a flash memory and in other types of reprogrammable non-volatile memory can leak. If the charge is not topped up from time to time the memory will lose its state.
In most consumer electronics applications devices are expected to be used in a relatively narrow band of temperatures around room temperature: for example from 0° C. to 40° C. However, in automotive applications and in some industrial and military applications a much wider range of operating temperatures is demanded. For example, some automotive manufacturers require that products operate reliably up to 105° C.
It is desirable to be able to use a general-purpose electronics product even in applications that call for a wide range of operating temperatures. A specialised high-temperature product is likely to be manufactured in lower volumes and therefore to be more expensive. FIG. 1 illustrates one scenario. FIG. 1 shows a wireless tyre pressure monitor. The tyre pressure monitor comprises a housing 1. The housing has a base 2 which can attach to a wheel rim with the opposite face 3 being exposed to the volume inside a tyre. A pressure sensor 4 is mounted in face 3. Inside the housing is a processing and communications integrated circuit (IC) 5 and a flash memory 6. The IC 5 is connected to an antenna 7 for transmitting and receiving radio signals and to a battery 8 which powers the device. In operation the IC 5 takes pressure measurements from the sensor 4 and transmits them to a receiver in the body of a vehicle. The memory 6 stores information such as communication parameters for the IC's interface to the vehicle. The IC could be used in many other applications, and by using a multi-purpose IC in this device the cost of the tyre pressure monitor can be reduced. With this in mind, in order to permit the tyre pressure monitor to operate up to, for example 105° C., a conventional approach is to use a multi-purpose IC and to select for the memory 6 a special flash memory device that has a low rate of decay at high temperatures. However, this causes other difficulties. First, such memory devices are more expensive than normal consumer-level memory devices. Second, they can require more energy than consumer-level memory devices for writing and reading. This is especially significant for a device such as a tyre pressure monitor because its battery cannot easily be changed.
Some memory devices provide the ability to measure the remaining retention margin, for example by varying the supply voltage. With such devices it would be possible to periodically measure the margin and refresh the contents when there is little margin left. This approach has two disadvantages. First, energy must be regularly expended in polling the device. Second, in environments where the temperature fluctuates the relationship between net margin loss and temperature may be difficult to estimate, meaning that the effective margin for a given device is difficult to establish based on a knowledge of reported retention margin and temperature.
There is a need for an improved way of providing memory that is tolerant of elevated temperatures, especially in very low power devices.